1. Field of the Invention
The present invention relates to a semiconductor device including a thin film transistor (TFT) and a device manufacture process thereof, and a contact structure (or a connection structure) of the wiring line in a semiconductor device and a contact forming process thereof. The semiconductor device of the invention includes not only an element such as a thin film transistor (TFT) or a MOS transistor but also a display device including a semiconductor circuit composed of such insulated gate type transistors, and an electrooptical device such as an image sensor. In addition, the semiconductor device of the invention includes an electronic device having such display device and electrooptical device.
2. Related Art
The thin film transistor (as will be called the “TFT”) can be formed over a transparent glass substrate so that its application and development to the active matrix type display device has been positively proceeded. In the active matrix type display device, an electric field to be applied to liquid crystals is controlled in a matrix shape by a plurality of pixels arranged in the matrix shape thereby to realize a highly fine image display. The TFT utilizing a crystalline semiconductor film can achieve a high mobility so that a highly fine image display can be realized by integrating functional circuits over a common substrate.
The active matrix type display device requires more TFTs as many as one million for the pixels as the resolution of the screen becomes finer. If the functional circuits are added, the more TFTs are required to retain the reliabilities and stable actions of the individual TFTs so that the liquid crystal display device may stably act.
The specifications required of the actual liquid crystal display device (as also called the “liquid crystal panel”) are so strict that both the pixels and the drivers have to retain high reliabilities for the normal operations of all pixels. Especially when the driver circuit is troubled, the pixels of one column (or row) cause malfunctions to invite a defect called the “line defect”. If one pixel is troubled, on the other hand, this trouble is called the “point defect”.
Most of the aforementioned line defect and point defect are caused by the malfunctions of the TFTs.
On the other hand, the material used for wiring these TFTs is exemplified by Al, Ta or Ti, of which aluminum having a low resistivity is frequently used. However, aluminum has a low resistance but has a drawback of a low heat resistance.
When the TFT is manufactured by using aluminum as the material for the gate line (including the gate electrode), a projection such as the hillock or whisker has been formed by the heat treatment to cause the malfunction of the TFT or lower the TFT characteristics. On the other hand, the aluminum atoms are caused by the heat treatment to diffuse into a gate insulating film and a channel forming region to cause the malfunction of the TFT or lower the TFT characteristics.
On the other hand, our Unexamined Published Japanese Patent Application No. 7-135318 which corresponds to a U.S. Pat. No. 5,648,277 has disclosed the TFT structure in which the periphery of a gate line is protected with an oxide film (or an alumina film) by using a thin film (as also called the “aluminum alloy”) composed mainly of aluminum as the gate line. When the TFT structure of the Japanese Patent Application is adopted, it is possible to prevent formation of the projection such as the hillock or whisker, but it is difficult to remove a barrier type anodized film (or an anodized film using a neutral electrolyte) formed over a gate electrode and to form a contact between the gate electrode and a lead-out electrode. Therefore, a malfunction is invited by the poor contact between the gate electrode and the lead-out electrode. In the peripheral drive circuit, on the other hand, a temperature rise or the like is caused by a large-current operation so that a reliable contact is also demanded.
On the other hand, the contact can be formed by using a special etchant, as called the “chromium-mixed acid (an etchant prepared by mixing an aqueous solution of chromic acid, phosphoric acid, nitric acid, acetic acid and water) as one for selectively removing only the barrier type anodized film formed over the gate electrode. However, the process using a heavy metal chromium, as may be detrimental to human bodies, is not industrially desirable. On the other hand, the etchant to take place of that chromium-mixed acid is not found at present.
On the other hand, the TFT utilizing a poly-silicon film is still inferior in the reliability to the MOSFET (a transistor formed over a single-crystalline semiconductor substrate) to be used in the LSI. So long as this weak point is not eliminated, moreover, there has been widely accepted a viewpoint that it is difficult to form the LSI circuit with the TFT.